Gas diffusion electrodes, as used in fuel cells and metal-air batteries, are generally made by mixing together finely divided particles of catalyst and a hydrophobic material, such as Teflon (brand of polytetrafluoroethylene, PTFE, manufactured by E. I. DuPont DeNemours & Co. (Inc.), Wilmington, Del. U.S.A.). The mixture is then deposited on a gas-permeable, conductive substrate to form the electrode structure. In this structure, the PTFE particles provide a hydrophobic porous matrix which will not be wetted by the electrolyte and through which diffusion of gaseous reactants and products can occur. The catalyst, ofttimes deposited on a catalyst support such as graphite or carbon, provides a hydrophilic porous matrix which is wetted by the electrolyte and through which the diffusion of dissolved reactants and products and also the migration of ions can occur. It should be recognized that the terms "hydrophobic" and "hydrophilic" as used herein denote wettability with respect to the electrolyte and are not intended to be limited to mean wettability with respect to water. In the operation of the electrode, the gaseous reactant diffuses through the PTFE matrix, dissolves in the electrolyte which fills the catalyst agglomerates and diffuses to the surface of the catalyst where the electrochemical reaction takes place. The electrons transferred during the chemical reaction are then conducted from the catalyst particle or its support to the conducting substrate.
As a result of the method of construction, the size, shape and orientation of the various particles and the flow channels formed therebetween are random. In some cases catalyst particles are fully insulated from the conducting substrate. Any catalyst so located will be wasted since, in the absence of a current-conducting path between the catalyst and the substrate, no chemical reaction can occur. Also, during the mixing operation, catalyst particles tend to agglomerate. Formation of too large agglomerates will result in waste of the catalyst since the center of the catalyst agglomerate receives an inadequate supply of the reactant gas. Formation of too small agglomerates will result in poor ionic and electronic conductance within the electrode structure. According to this invention, however, an electrode structure is provided in which clearly defined and controllable gas and liquid passages are provided and in which the catalyst can be located at the interfaces therebetween in a manner producing maximum catalyst utilization and minimum resistance to the flow of reactants, products, ions and electrons.
It is accordingly an object of this invention to provide a novel electrochemical cell electrode structure having clearly defined hydrophobic and hydrophilic passages.
It is another object of this invention to provide a method of manufacture of such an electrode structure.